Son of Kong

The story picks up about a month after the dramatic finale of the previous film and follows the further adventures of filmmaker Carl Denham, now implicated in numerous lawsuits following the destruction wrought by Kong. Carl Denham leaves New York City with the captain of the Venture, Captain Englehorn, who is certain it is just a matter of time before he is similarly served. Their efforts to make money shipping cargo around the Orient are less than successful. In the Dutch port of Dakang, Carl Denham is amused to see there's a "show" being presented, so he and Captain Englehorn attend. It turns out to be a series of performing monkeys, capped by a song ("Runaway Blues") sung by a young woman named Hilda Petersen.
That night, Hilda's father, who runs the show, stays up drinking with a Norwegian skipper named Nils Helstrom, who had lost his ship under questionable circumstances. The two men fight and Hilda's father is killed, their tent burns down and Hilda releases all the monkeys. Carl Denham and Englehorn run into Helstrom, who was the man that sold Carl Denham the map to Kong's Island, and he convinces the two that there was a treasure on the island. Carl Denham and Captain Englehorn agree to go back and try to retrieve it. Later, Denham meets Hilda while she is trying to recapture her monkeys and tries to cheer her up. Despite her pleas, Carl Denham refuses to take her with him when he leaves Dakang. Shortly after they put out to sea, however, Hilda is found stowing away on board.
Helstrom talks Hilda into silence and incites a mutiny on board the Venture, but the sailors want no more captains and throw him overboard alongside Denham, Englehorn, Hilda and the cook, Charlie. The five land on Kong's Island where they discover the natives blame Carl Denham for the destruction of their village and they are forced to move to a different part of the island. There, Carl Denham and Hilda Petersen meet and befriend an albino gorilla just over twice the height of a man. Carl Denham assumes the ape to be Kong's son and names him "Little Kong". Meanwhile, Captain Englehorn, Charlie and Helstrom are attacked by a Styracosaurus which chases them into a cave. Denham and Hilda are attacked by a giant cave bear but Little Kong fights and fends it off by swinging a tree branch. Carl Denham bandages Little Kong's injured finger in return. Despite the fact that Helstrom made up his story out of desperation, Carl Denham finds an authentic treasure. Shortly afterwards, Little Kong, Carl Denham and Hilda Petersen are attacked by a dragon-like Nothosaurus which Little Kong kills, while Helstrom tries to escape in the lifeboat but is killed by a Cetiosaurus. Hilda Petersen, Captain Englehorn and Charlie run to the lifeboat, but an earthquake strikes the island and it begins to sink into the ocean. Little Kong has his foot stuck on the top of a mountain of the island, and he sacrifices his life by saving Carl Denham by holding him above the water until he can be rescued. The film ends with Carl Denham and Hilda Petersen throwing their lot in together, as the treasure will make all four of them (including Captain Englehorn and Charlie) wealthy.

Question: Who sold Carl Denham the map to Kong's Island?
Answer: Helstrom