Gameplay
Tomb Raider: Underworld is a single player, action-adventure video game. The game is presented in third person perspective, where the player takes control of Lara Croft. Lara's environment reproduces a more "interactive world that reacts and remembers", such that footprints left in the mud or mud transferred to Lara's knee from kneeling on the ground is washed away by rain, the bodies of the foes she encounters remain where she killed them, and any destruction to the environment she causes is permanent. According to creative director Eric Lindstrom, this is "to not only reward the player for the effect they're having on the world, but to give them navigational aids." The game uses an animation blending system that allows Lara to interact dynamically with her environment, such as pushing foliage aside with one or two hands, depending on if she is carrying a weapon. It also features a "hybrid lighting model that combines dynamic lights with carefully created light maps" and a weather system that changes the environment, for example, "If Lara’s negotiating a wet ledge she’s more apt to slip or lose grip," which makes "the environment ... her adversary" for a large part of the game.
Lindstrom explained that they integrated the elements of climbing, shooting and puzzle solving. This instalment also features a new melee combat system, requiring Lara in some instances to use "direct combat and evasive manoeuvres to distance herself from her attacker". Notably, Lara's bike, among other things, is a key component in solving the puzzles she will encounter in her adventure. Pick-ups have multiple uses as weapons and tools in interaction with the environment, and Lindstrom stated that Lara "can also split up her guns and fire at two different targets simultaneously," or hold an item with one hand and fire a gun with the other. The grappling hook can be stretched taut and used to pull objects off ledges, illustrating what project lead Rob Pavey said, that "Lara will be able to do anything that you'd expect her to be able to do," which he called "the big theme this year." Lindstrom describes this as "a philosophy called 'What Could Lara Do?'—WCLD. It's short-hand for having the player be able to use their own intuition about what someone with her abilities should be able to do in an environment such as this, and consistency across the different mechanics and abilities. If she can throw a grenade, then if she can pick up this pole, why can't she throw it?" Crystal Dynamics also made the game non-linear, offering elaborate multi-stage puzzles.
Another new design element was the "adrenaline moments". Instead of specific button presses, time slows down and gives the player a chance to get out of harm's way while retaining complete control of Lara.
Plot
Tomb Raider: Underworld begins with Lara Croft's mansion exploding and being engulfed in flames. The game rewinds back in time to a week before the explosion, just after the events of Tomb Raider: Legend. Lara is searching for Avalon, hoping it will lead her to an explanation for the disappearance of her long lost mother. Underneath the Mediterranean Sea, Lara discovers an ancient temple designating itself as "Niflheim", one of the many Norse underworlds. Lara recovers one of Thor's gauntlets after a lengthy battle with Amanda Evert's mercenaries, and an encounter with an imprisoned Jacqueline Natla on Amanda's ship. Natla tells Lara that the Norse underworld, Helheim and Avalon are one and the same and that she will need to find Thor's Hammer to open the Underworld and find her mother. Lara discovers that she will have to find Thor's other gauntlet and his belt if she wants to find and wield the hammer. Natla provides Lara with a starting point for her search in this quest – coastal Thailand. In Thailand, Lara does not locate the next gauntlet, but she finds evidence that her father had retrieved it before he died. She also discovers a message that reveals her father and Natla had a working relationship which eventually soured. Lara is able to deduce where her father hid the missing gauntlet.
Back at the Croft Manor, Lara finds her father's secret office buried beneath her home. Upon his desk, Lara discovers the gauntlet as well as a tape-recorded message, warning her that Helhiem contains a powerful weapon. An explosion occurs and Lara's home becomes engulfed in flames, leading back to the opening events of the game. Zip tries to shoot Lara and claims that "Lara" detonated the bomb. When Lara returns to the burning office to recover the security footage, she encounters her doppelgänger who kills Alister Fletcher. After his death, Lara resolves to continue with her quest. In Mexico, Lara finds both Thor's Belt and ancient pictographs linking the weapon in Helheim to Jörmungandr, a mythical Norse sea-serpent, brought about by the seventh age. Her next stop are ruins on Jan Mayen Island that correlate to Valhalla. It is here that Lara recovers Thor's Hammer. In the meantime, Zip has managed to track Amanda down to a sister-ship of the one Lara sank earlier. Armed with Mjolnir, Lara boards the vessel and interrogates Natla once again. Natla provides Lara with the coordinates of Helheim, but points out that Lara does not know the Ritual of Odin, which is needed to open its gates, so Lara reluctantly strikes a bargain with Natla and frees her from her cell.
They rendezvous in the outer chambers of the Helheim complex, deep below the Arctic Sea. With the ritual performed, Lara is able to use Mjolnir to open the gates of Helheim. Along the way, Lara discovers the horrifying truth of her mother's fate – she has been turned into a thrall, thus Lara forcibly shoots her. Natla reveals the true extent of her manipulation of Lara, also revealing that she was responsible for killing Lara's father. The doppelgänger prepares to kill Lara, but she is saved by Amanda. It is revealed that the Midgard Serpent was a Norse metaphor for the many tectonic divisions that encircle the world, beneath the seas. The doomsday device was built upon the most unstable junction of these lines and its activation would cause massive volcanic activity across the whole planet and the destruction of most of humanity. Lara successfully destabilizes the device and strikes Natla with Mjolnir, sending her down into the pool of deadly eitr below. Lara and Amanda escape together using the dais, like the one that brought Lara's mother to Helheim, teleporting back to the temple in Nepal (from Tomb Raider: Legend). They go separate ways from there, and Lara moves on to new adventures.
