She woke up with a man’s hand pressing down on her nose and mouth. She couldn’t breathe, couldn’t see in the pitch-black room. At first she thought, “It must be my brother, coming home late.” Then Shea McDonough, 16, heard a voice say, “If you make any noise, I’ll f—— kill you.” There was no face—just a black mask, like a ninja’s, with holes cut out for the eyes. And there was a hunting knife. “He’d pulled the covers down to my knees,” says Shea. “That’s when I started panicking.”

What happened next, in a few desperate moments in a quiet Massachusetts neighborhood, is at once the stuff of nightmares and a story of incredible heroism. Shea’s frantic thrashing and a single muffled cry, just before 4 a.m., roused her parents, Kevin and Jeannie, who shuffled in to check on her, thinking she’d had a bad dream. Instead they saw the hulking intruder and, together, somehow managed not only to pry him off their daughter but, in a bloody, violent fight, subdue him until police arrived. They soon learned the man they improbably captured last July 30, Adam Leroy Lane, 43, was a suspected serial killer who had stabbed another woman to death less than 24 hours before attacking Shea. “What they did that night is unbelievable,” says Chief James Murphy of the Chelmsford Police Department. “Sometimes people freeze, but they reacted. With any hesitation, things would have turned out differently.”

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That they came so close to a different ending—what if they hadn’t heard Shea? What if Lane had overpowered them?—still haunts the McDonoughs, who agreed to share their story only after Lane pleaded guilty to the attack and received a sentence of 25 to 30 years in prison on Dec. 11. “I still watch Shea like a hawk,” says Kevin, 49, a utility line contractor, who had a new alarm system installed in the family’s four-bedroom Cape on a leafy street in Chelmsford, 30 miles northwest of Boston—a town that was voted one of America’s best places to live by Money magazine in 2007. “We’d always say, ‘Come on, what could happen in Chelmsford?'” says Jeannie, 47, an office administrator. “I never, ever thought someone would be lurking outside my door.”

Virtually everyone in Chelmsford was asleep on the sticky, hot night of July 29 when Lane parked his truck at a rest stop and walked about a mile into town. A driver for a tree nursery who lived in Jonesville, N.C., with his wife, their daughter, 7, and two daughters from her previous marriage, Lane slipped two knives into sheaths on his legs and, into a fanny pack, stuffed a black leather mask, a sharp-bladed weapon called a throwing star and a choke wire. In the truck he left the tape of a gory movie about a serial killer. The title: Hunting Humans.

Lane spent hours creeping along streets and peering through windows; two residents called police to report a figure lurking around their homes. Around 4 a.m. Lane tried the back door of the McDonoughs’ home, found it unlocked and walked right in. Their black Lab-retriever mix, Boscoe, was outside and never once barked; they believe Lane may have drugged him.

Lane made his way past the master bedroom, where Kevin and Jeannie were sleeping, and into the guest room. Fatefully, Shea wasn’t in her own bedroom, which is much farther away from where her parents sleep, but had settled in the guest room at 1 a.m. because it was air-conditioned. (Her brother Ryan, 19, was at a friend’s house that night.) Suddenly Shea felt “this really big hand over my mouth.” A swimmer with strong legs, she kicked “and tried to push against the wall, so my parents would hear.”

They did—but just barely. Her mother, Jeannie, heard “this weird little whimper and a bump on the wall.” Kevin heard it too and said, “I’ll go check,” and Jeannie said, “No, I’ll go,” and so they went together. Says Jeannie: “We never both go.”

When they opened the door, they saw Lane, 5’11” tall and some 245 lbs. Without thinking, Kevin, 5’9″ and 160 lbs. and wearing only boxer shorts, lunged at Lane, grabbed both his wrists, pushed him down and lay on top of him. “Who are you?” Kevin screamed, while Jeannie, 5’1″ and 135 lbs., tried to pry the knife out of Lane’s hand. Suddenly Lane “got this burst of adrenaline and stood up with me on his back,” says Kevin. “In my head I said, ‘Oops, I’m in trouble here.'”

Yet Kevin held on and got his arm around Lane’s neck; both men crashed backward into a wall. “I had his windpipe and kept yanking on it as hard as I could,” says Kevin. Meanwhile Jeannie wrestled the knife away, badly slashing both her hands. By then Shea was up and dialing 911 on her cell. It seemed like hours to the McDonoughs, but within four minutes police arrived. “A big guy named Murphy came in and said, ‘Don’t move or I’ll blow your head off,'” says Kevin, who calmly brushed himself off and said, “Get this scum outta here.”

The McDonoughs later learned that Lane was responsible for at least one murder and is linked to several others (see box). Since the attack they have received a slew of cards from well-wishers, including one from the loved ones of one of Lane’s apparent victims. “If it was not for your family,” the note reads, “we may never have known who killed our best friend.”

Things are getting back to normal—slowly. Shea has been seeing a therapist and has made progress in handling the terror she sometimes feels. But she still has trouble sleeping alone (Boscoe now sleeps beside her). “Every little shadow I see,” she says, “I sit right up.” Even so, she is grateful to be alive—and somewhat in awe of her parents. “Their will to fight is what I remember most,” she says. “I remember my mom flying around in her underwear and tank top.” Kevin and Jeannie dismiss the idea that they’re Super Parents—they say they acted instinctively to protect their child. “To this day I don’t know how I got that big guy on his stomach,” says Kevin. “I felt like Hercules.”