The thing in the gasmask is a demon known as Sun Moon Stars who is stalking Murdoc (The ugly mofo with the tongue) in order to claim his soul (Which Murdoc sold to him at some point in the past in exchange for musical talent).
Murdoc has sequestered himself on an abandoned island made out of plastic products with his former bandmate 2D (The fellow in the clownmask) and a robot replica of other bandmate Noodle (The girl with the guns) as well as an assortment of celebrity guests
2D is housed in a room underwater where he is plagued by a whale lurking outside the porthole of which he has an intense phobia.
Meanwhile, former bandmate Russell (The gigantic fellow) was enraged upon discovering Murdoc had not called him to plastic beach and decided to swim there himself to enact his vengence. Along the way Russell ingested large amounts of pollution which has inexplicably caused him to grow to gigantic size.
During his voyahe to plastic beach Russell encountered the real Noodle )The girl in the dog mask) who had been marooned in a lifeboat after her cruise ship was attacked by pirates. The two, in this video, arrive at Plastic Beach and the real Noodle confronts her robotic replica.