Serenity is a 03 Firefly-class spaceship featured on the science fiction television series Firefly and follow-on movie Serenity, created by Joss Whedon. She fits the profile of a light freighter, having a cargo area equivalent to about three or four semis. The photon-reaction drive in the bulbous rear section of the ship lights up when accelerating, resembling a firefly and, thus, the class moniker. The engine pods rotate down like a VTOL aircraft. Whenever the ship lands, her wings fold down like the wings of a bird coming to rest. The aft of the ship is outfitted with extenders beside and below the photon-reaction drive to ease the turbulence of lift-off and atmospheric re-entry. As she is a run-of-the-mill civilian cargo vessel, Serenity has no weapons or defences beyond the guile and skill of her crew.

Serenity's layout is triple-tiered. The upper deck houses the cockpit at the front, in the "head". Moving aft from the cockpit leads into a corridor through Serenity's "neck", in which the crew quarters are housed. The starboard quarters house the rooms of ship's mechanic Kaylee Frye and captain Malcolm Reynolds, and the port quarters house the rooms for mercenary Jayne Cobb and married couple Zoë and Hoban "Wash" Washburne, the first mate and pilot, respectively, of Serenity.

There is a passage in this corridor leading down to the upper cargo bay catwalk. The middle of the top deck, in Serenity's "thorax", is the galley and kitchen, hued in a gentle yellow and decorated in part by Kaylee's paintings of flowers. Aftwards of the galley, deep in Serenity's "abdomen" and accessible through the back of the galley, is the engine room, housing the Firefly drive and a great deal of mechanical interfaces. It also houses Kaylee's hammock.

The flight of stairs attached to the aft corridor leads down to the ship's common area, part of which is taken up by an enclosed medical bay. The common area is spacious, and furnished with settees and sofas; the medical bay is (usually) kept clinically clean. Aft of the common area and medbay are the passenger quarters, perhaps higher-quality than that of the crew's. They are of traditional Asian design, with simulated-rice-paper, sliding doors made of light diffusing plastic. Regular occupants of these quarters include doctor Simon Tam and his sister River, as well as Shepherd Derrial Book.

Forward of the common area is the cargo bay. A vast, open space, two decks tall, it is the largest single room on the ship. Its upper catwalk is accessible via two flights of stairs. The front of the bay houses an airlock; the two inner lock doors slide in from either side, whilst the outer lock door rotates down and out, forming a convenient ramp when lowered. There are controls to operate the airlock both within the lock itself, and on the inside of the cargo bay. The outer lock door also has a door-like component in its centre, allowing personnel to enter and exit the ship without needing to open the ramp fully; however, for vehicular access or access for large cargo, the ramp must be lowered. There is a second airlock / cargo ramp directly in the centre of the cargo area. It is used for a disposal / drop bay, but can be used to lower a small cargo elevator while the ship is hovering.

In "Bushwhacked", we learn through dialogue that the Firefly transport has "troublesome little nooks," that garners favour with smugglers. In that respect, the Firefly is not unlike the Millennium Falcon from the Star Wars films; both are aging transport models re-engaged into service as smuggling vessels for anti-hero captains. In the episode "Serenity", cargo 'liberated' from a derelict transport is hidden in these nooks, to avoid awkward questions from the passengers the crew were about to take on. Onboard the Millennium Falcon, the crew has to hide in the smuggling compartments when the ship is pulled into the Death Star by a tractor beam. Stormtroopers board the ship looking for the crew, but are unable to find them.

The cargo bay also houses the "mule", a small local-transport vehicle. In the television series, this was effectively a yellow quad bike modified for cargo hauling. This was destroyed in the episode "War Stories" after being set on fire, and was replaced by a hovering vehicle sometime before the main events in the film Serenity.

The upper catwalk also provides access to the ship's two shuttles. The port shuttle is general-purpose, and used for hauling cargo, escape purposes, or as an alternative means of transportation. The starboard shuttle was rented out to Companion Inara Serra during the television series. Both shuttles are extended out of the ship before launch, and drawn in when docked.

The main defence thought up by Serenity's crew is a little device called the "cry-baby". Housed in a coffee can, the cry-baby gives off a radio distress signal and electromagnetic signature that mimics other spacecraft, requiring scanning pursuers to visually inspect the device to uncover the deception. The cry-baby can even be outfitted with small, one-shot thrusters to give it the appearance of being a ship in flight. The crew uses cry-babies to get out of trouble with the Alliance on more than one occasion.

The ship Serenity underwent some changes for the film Serenity, most obviously in the internal arrangement of the cargo bay. The entire rear wall has been replaced by a large, circular-patterned construct, presumably a structural link to the large rear engine in the "abdomen". A great deal of bracing has been added to the side walls, which themselves seem to be more circular than the angular walls seen in the series. The bomb bay airlock has been altered, in that the two inner lock doors have been replaced - they no longer rotate into the cargo bay, but instead lower slightly, then retract sideways into the floor. Finally, the forward airlock has also been replaced. The forward door no longer rotates outward into a ramp; rather, it is brought down and into the ship, sliding neatly underneath the airlock floor. This obviously has the advantage of allowing the airlock to open fully in more constrained conditions, which would have prevented the old ramp from opening fully.

The common area of the ship has seen few changes. The only major differences are that the circular structural construct in the cargo bay is mirrored on the forward wall of the common area; additionally, the portside wall is now effectively made out of storage spaces.

On the upper deck, the engine room is now home to a great deal more of piping, tubing, and other mechanical interfaces. The rear corridor is now lower than it was in the series as the stairs in and out of the galley to the rear corridor now lead down into the corridor, rather than up to it, as they did in the series. The galley's octagonal sitting area now has extra bracing, as well as seats with security straps to deal quickly with turbulence and rapid acceleration or deceleration of the ship. The forward corridor, again, has been braced more securely, with heftier-looking support struts. The ladder-doors now open more gently, seemingly outfitted with a cushioning mechanism. The bridge has seen few changes, but the forward flooring section has been entirely removed, exposing an extra cargo-storage section in the nose of the ship.

The exterior of the vessel is also visually different. It has a new paint job; decked out in greyer and bluer colours than the bronzed Firefly of the series (many believe this to be deliberate on Joss and Zoic's part, as Serenity's interior and exterior colour schemes change throughout the film to reflect the characters and the story). The jet engine cowling has apparently fallen off, or been lost in some other way. Finally, the struttish landing legs of the series have been replaced by legs of a more organic design, with better cushioning and claw-like 'toes'.

Serenity was acquired by Mal in 2512, shortly after the end of the Unification War. Her name comes from one of the most decisive battles of the war between the Alliance and the Independents, which took place in Serenity Valley on the planet Hera. Mal and his first officer, Zoë, wanted to be free of the Alliance and having a ship was the best chance. At the time it was purchased, Serenity was little more than a planet-bound derelict. She was still intact, still space-worthy, but her engines were shot and most of her equipment was in sore need of repair. Mal's plan was to acquire a pilot, mechanic, and cook and start taking jobs immediately. It took them almost a month to get Serenity off the ground, as her first mechanic, Bester, was not as advertised. Mal was quick to realize how hard it was to find work in an Alliance controlled solar system, and is not above using Serenity for ill-gotten gain if the gain is high enough.

In the film, the crew disguise Serenity as a Reaver ship (in order to sneak through Reaver space) by essentially mutilating her; tearing holes in the hull, adding weapons and strapping skeletons to the bow. Later in the film, Wash is manoeuvring Serenity through an air battle between the Alliance and the Reavers when the ship is struck by a Reaver's EMP weapon. With main power down, Wash is barely able to glide her into a controlled crash landing on the landing strip of Mr Universe's complex; the landing gear is crushed, an engine pod is severed, and part of the main engine is destroyed before the ship comes to a complete stop. At the very end of the film, the whole crew conduct major repairs and are able to fix Serenity.