Helen is delighted to learn that an old cryptography machine in her care has sprung to life, but when she deciphers the machine's message, the mood turns serious: Nicola Tesla is telling her he's in distress. Will warns Magnus that Tesla must have an ulterior motive, but she insists that they trace the signal of his call. They find him deep in an underground cave system in Colombia, wrapped in the webbing of a strange insect. When those creatures giant millipedes discover Helen and her team on their turf, they get angry. The Sanctuary gang has no choice but to run, locking them even deeper in the catacomb system. Tesla's true ambition soon becomes clear, however, as they stumble onto a wrecked and abandoned Cabal laboratory.
Back at the Sanctuary, Bigfoot learns the shocking news that one of his oldest friends the man who introduced him to the Sanctuary has been murdered, and he goes on the hunt for the culprit. His trail leads him to a street thug, who convinces Bigfoot that he's just a patsy and Bigfoot knows he's telling the truth. Believing he was set up by someone on the police force, Bigfoot and Kate set up a trap with the thug as bait. Like clockwork, Detective O'Farrell, the lead on the murder case, arrives and nearly shoots the thug, but the Big Guy drops him and asks him what's going on behind the scenes. O'Farrell says he had no choice, and as a cloaked figure appears on a nearby rooftop, then disappears as if by magic, the detective writhes in agony, then dies.
Tesla attempts to be enigmatic in the ruined Cabal lab, but Magnus, Henry and Will soon deduce that he was after the source blood stored there, desiring to turn himself back into a vampire. Although they're fuming that he would drag them into danger for his selfish purpose, they have a more pressing concern: The giant insects, which have mutated into abnormals thanks to the exploded source blood lining the cave walls, are now on the hunt and Helen and her staff are sitting in the nest of their young, where hundreds of pupae are ready to hatch. Amid groans of protest from Tesla, Henry finds a way to build a bomb with a sixty-second delay, long enough for them to clear out before destroying the abomination of nature that is the race of bloodthirsty insects. They rig the explosive, but Tesla hesitates, and it's long enough for the bugs to go on the attack. Tesla sacrifices himself, and uses his super magnetic powers to shut the mighty iron door, shielding the others from the blow. When the dust clears, Helen and the boys discover that Tesla has used his power a second time, to burrow himself in life-saving scrap metal.
Back at the Sanctuary, Helen comforts the Big Guy, and tells him that the autopsy of the detective came back completely normal, but Bigfoot knows something's not right. What that thing is, however, remains a mystery.