Co-written and directed by William Friedkin (The French Connection, The Exorcist, Sorcerer, Cruising, To Live and Die in L.A., Jade, Rules of Engagement, The Hunted, Bug, Killer Joe), The Guardian opens with an explanation regarding ancient druids and how they worshiped trees, sometimes offering them human sacrifices. It explains how many of these trees were connected with evil. Whilst this appears to be quite random, it does come up later in the plot!
We then get to meet Molly (Natalia Nogulich) and Allan Sheridan (Gary Swanson) who are heading off on a trip, leaving their two children in the care of their nanny, Diana Julian (Jenny Seagrove). Once alone and with the children sleeping, Diane kidnaps the daughter.
Luckily Molly realizes she left her glasses at the house, and upon returning she sees that her daughter is missing. Diana, out in the forest, approaches a giant, old, gnarled tree, and holds up the baby, which then disappears from her hands. The roots of the tree show the baby’s face, as it has been sacrificed to keep the tree alive. As Diana sits on the ground near a pool of water, her reflection disappears, leaving that of a growling wolf.
Three months later, Phil (Dwier Brown) and Kate Sterling (Carey Lowell ) move to Los Angeles, where Phil has gotten a job with an advertising agency. Kate reveals to Phil that she is pregnant; she gives birth to a baby boy, Jake. They decide to hire a nanny so that both parents can continue to work. They interview a number of candidates recommended by the Guardian Angel agency. Two seem most suitable: a young woman named Arlene Russell (Theresa Randle), and a caring British woman named Camilla Grandier (Jenny Seagrove).
After Arlene dies in a bicycling accident, Camilla is hired and quickly becomes an invaluable member of the household. However, one night, Phil has a nightmare where he sees a wolf attacking Jake. Later in the day, Camilla and Jake are outside in a field when she’s confronted by three gang members (Jack David Walker, Willy Parsons, Frank Noon) who chase her and Jake into the woods. After knocking her to the ground, one of the men stabs her and attempts to rape her at the base of the old tree. The tree comes alive and uses its branches to eviscerate the three men, consuming one of them. Camilla sits on a branch above the ground, cradling the baby, while wolves devour one of the men. A giant spike impales the last man, before he is engulfed in flames…