For most of the game the player will be in control of Ratchet but in some areas the player takes control of Clank, Ratchet's ally. Clank can give commands to smaller robots called gadgebots to perform tasks with the use of a command menu to solve puzzles and attack enemies.Clank also features in "Giant Clank" scenarios where Clank transforms into a larger and more advanced form and can battle similar size enemies; in this form he gains increased fire power and a large amount of health. During the single-player game, the player can also play as Captain Qwark in 2D side-scrolling levels (called "Vid Comics"), which build upon the history of Captain Qwark and his previous conflicts with Dr. Nefarious.
The game offers a huge range of enemies and different stages. It also features online multiplayer modes, such as Deathmatch, Capture The Flag, and Siege (in which the player storms the opponent's base and tries to destroy the power core inside). It supports up to eight players at a time and supports voice chat through a PS2-compatible headset. Ethernet connection is needed if playing online against other players.
The player character and main protagonist is Ratchet (voiced by James Arnold Taylor), a Lombax from the planet Veldin. He wields a wrench as a melee weapon and can use a large arsenal of weapons, including guns, and melee weapons. Clank, Ratchet's robotic sidekick and sometimes co-protagonist, is usually attached to Ratchet's back and uses attachments to help Ratchet's movements. He is voiced by David Kaye. Dr. Nefarious, the main antagonist of the game (voiced by Armin Shimerman), is the robotic form of a former organic life form. He has a hatred for organic life and tries to destroy it. At the end of the game, Doctor Nefarious is left stranded on an asteroid, eventually making cameos and reappearances in later titles.
The game opens with Ratchet and Clank finding a news report about Ratchet's home planet, Veldin, which is under attack by an army of aliens named Tyhrranoids. The lead suspect is a mysterious villain by the name of Dr. Nefarious. After they help to defeat the Tyhrranoids on Veldin, the Galactic President tells Ratchet that Captain Qwark, an enemy from the first two games in the series, is the only person to ever have defeated Dr. Nefarious and that Qwark's help is needed to defeat him again. Ratchet finds Qwark and takes him to the Starship Phoenix, whose commander has been ordered to help Ratchet and Qwark defeat Dr. Nefarious. Qwark creates the "Q-Force", a group of "elite" people working against Dr. Nefarious, and he assigns Ratchet a new mission to infiltrate Nefarious' base to acquire as much information on Nefarious' plans as possible. They find that Dr. Nefarious is a fan of Secret Agent Clank, Clank's TV show, along with an encrypted star map to the Tyhrranoids' home planet. After the Q-Force destroys a Tyhrranoid base on their home planet, Dr. Nefarious sends the Q-Force a transmission telling them that they won't be able to defeat him, and Al traces the transmission to planet Daxx.Despite straying further from a classic platform game, Ratchet & Clank: Up Your Arsenal still offers some of the basic features of a platform game but with a strong mix of action and shooting; PGNx Media has dubbed it a "combat-platformer". It offers melee and long range combat and boasts a large, varied arsenal as one of its primary features as it has been in previous games.Ratchet is initially armed with an Omniwrench 8000, his primary melee weapon, but as the player progresses through the game new weapons and upgrades can be obtained. As well as the 20 weapons available to the player, each weapon (apart from the RY3NO) has eight maximum levels of strength, which enhances on the previous game's progress of only leveling up once, five are available after weapon purchase and then three after that once the game has been completed and the player starts "challenge mode". Alongside the weapons, which are primarily used in combat, there are ten gadgets that can be collected for use in other situations.