In this movie sequel, Vin Diesel returns as Riddick, an escaped convict with a price on his head. Riddick has been hiding on a snow planet for the last five years, when a group of mercenaries try to capture him. Riddick returns to the planet Helion, and finds out that his friend Jack, is in prison on Crematoria, a very hot planet. While on Helion, the planet is invaded by the Necromongers, led by Lord Marshal (Colm Feore), who wants to rule the universe. Riddick is captured by the mercs and flown to the same unsavoury and possibly illegal prison Kyra is in. Turns out Jack is now known as Kyra (Alexa Davalos) and is tough as nails. The mercs and the prison guards can't agree on the bounty to be paid for Riddick. When the guards learn that a Necro ship has followed the mercs to the planet, tension boils over and the guards and mercs shoot it out. Following the battle, Riddick, Kyra, and the other prisoners escape. They realize the guards have deserted them and are attempting to escape the Necros; they head out across the surface of Crematoria hoping to beat the guards to the escape ship. Meanwhile, the Necro Dame Vaako (Thandie Newton) speaks with Aerion (Dame Judi Dench), an air Elemental capable of seeing into the future in some limited way but now imprisoned by the Necros. Aerion reveals that The Lord Marshal previously tried to exterminate the race of Furians because of a prophecy which foretold he would be killed by one. Riddick is a Furian. Back on Crematoria, Riddick and his fellow escapees race across the ravaged surface to stay ahead of the sunrise, as the surface termperature of the planet increases to 700F in full daylight, and fight the surviving prison guards on the way. They reach the escape ship only to find Necros and guards already there. A three-way battle ensues between the Necros, the convicts, and the guards; the guards are all killed, Kyra taken prisoner, and Riddick left for dead by Necro Commander Vaako. He is rescued from the sunlight by a Necro who was born a Furian like Riddick, who gives him a message from the Lord Marshal (in case he survived the attack): he will no longer be hunted so long as he stays away from Helion. The ex-Furian gives Riddick a Furian knife, then kills himself. Naturally, Riddick heads to Helion to take on the Lord Marshal and find Kyra. Believing Riddick dead, the Lord Marshal promotes Vaako to first among Commanders. The scheming Dame Vaako realizes Riddick is on Helion, and plots with Vaako to let Riddick attack the Lord Marshal and, if he is at all successful, finish him off and sieze power themselves. Aerion reveals to the Lord Marshal that Riddick is in all probability still alive. Riddick attacks the Lord Marshal. The Lord Marshal tells Riddick to join him, and reveals that Kyra has been turned to a Necro. Riddick fights him instead, and is nearly killed before Kyra - apparently not all Necro yet - spears the Lord Marshal in the back. The Lord Marshal returns the favour and impales her on a spike jutting from the wall. Seeing the opportunity, Vaako grabs an axe and swoops in to finish off the Lord Marshal, but Riddick kills him first with the Furian knife. Kyra, dying, tells Riddick she was always with him. Riddick collapses on the Necro throne, and they all bow to him, their new leader., Question: What is the relationship between Riddick and Jack?
Output: Answer: Friends