From Halopedia, the Halo wiki
The Life (referred to as We are ODST in the Xbox Live Marketplace) is a live-action short made exclusively for the launch of Halo 3: ODST. The film is directed by Rupert Sanders, who also worked on Halo 3's Believe campaign.
The film starts with a military funeral held for Staff Sergeant K. Stark, with a young Tarkov watching, and then shows his recruitment and basic training to enlist as a Marine. Later, the video skips to a combat drop as Tarkov, now an ODST in an insertion pod, dropping towards a Covenant-held battlefield on an unknown planet, where he soon encounters a Brute and is nearly killed, acquiring wounds to his face. The Brute closes in for the kill as Tarkov fires his pistol, but the Brute is killed by a crashing Banshee, saving Tarkov's life. The scene skips ahead to Tarkov many years later, the wounds on his face having become scars. He and a squad of fellow ODSTs hold a small service for a fallen comrade killed during combat, and hear a distant explosion and plasma fire in the distance. Forced to abandon the service, the ODSTs put their helmets on and move out.
Translated from Hungarian
The video starts at Stark's funeral proceedings. Tarkov is seen in the background. A rendition of Light of Aidan's Lament plays in the background from the beginning to the firefight
As the Marines raise their assault rifles, Tarkov visibly flinches.
Cut to Tarkov now having joined the ODSTs, having his head shaven by a barber. Fear is in his eyes.
Cut to the next scene where a group of ODST trainees stand in a file, then snap to attention. A drill instructor yells in front of Tarkov.
The scene changes to an obstacle course. Tarkov performs a commando crawl and a UNSC instructor follows him, firing an M6C/SOCOM pistol over his head, yelling.
As Tarkov takes a dive in the obstacle course, the scene cuts to him being dropped into an unknown planet. Several SOEIVs are seen descending to the surface. As soon as he hits the ground, his visor polarizes and his SOEIV's hatch bursts open. Tarkov, along with several other ODSTs, start running to an unknown destination, firing their SMGs. Along the way, they pass scenes of fallen ODSTs while under plasma fire, several ODSTs providing covering fire, and Banshees strafing the advancing ODSTs. One of the ODSTs in front of him is shot down as he continues forward while another wounded ODST is being dragged back by two comrades. However, a Brute Minor knocks Tarkov to the ground, knocking his helmet off and inflicting deep gashes in his face. The Brute growls at him as Tarkov fires his pistol to no avail. The Brute closes in for the kill as Tarkov fires his pistol, but the Brute is killed by a crashing Banshee, saving Tarkov's life. Tarkov, lying in the mud, stares at the open space.
Cuts to a makeshift battlefield grave site. Tarkov is now much older, with the gashes from the Brute still on his face as scars, and several other ODSTs stand alongside him. The service having ended, Tarkov folds up the ODST flag that had been placed on the makeshift grave and puts it into a storage compartment in his armor. Cut to a younger ODST who jerks his head up at the sound of distant plasma fire. Tarkov looks toward the source and puts on his helmet, which polarizes. The younger ODST does the same and the squad moves out. As the screen fades, the song repeats and becomes louder. The words "We Are ODST" appear, then change to 'Halo 3: ODST'.
The lyrics of the song in Welsh: "Gafflwn Dihenydd, o'r fuddugol yn wiriol sydd. Ni fydd neb yn ein Drechu, Falch ydy ni i drochu, Traed o flaen i'r Annwn, mewn y gwybodaeth fe godwn ni."
The translation of the original Welsh lyrics: "We cheat Death from his rightful victory. No one can defeat us. We are glad to plunge feet first into hell in the knowledge that we will rise."