A fourth quarter for the ages, a legendary dunk from Al Horford, and the Celtics-Bucks series is all even

Jayson Tatum delivered from all spots on the floor -- including this unique angle -- down the stretch to help lift the Celtics Monday night in Milwaukee.Jim Davis/Globe Staff

MILWAUKEE — If the Celtics go on to win Flag 18, years from now when tall tales are told, this might be the night folks talk about the most. This might be like Dave Roberts stealing second base, or Adam Vinatieri drilling the ice ball through the uprights in the last game played at Foxboro Stadium

The Celtics beat the World Champion Milwaukee Bucks, 116-108, at Fiserv Forum Monday night to square their conference semifinal series, 2-2. Game 5 is Wednesday at the Garden, Game 6 is Friday back in Brewtown, and Game 7 (probably necessary) Sunday on Causeway Street. Bring your popcorn.

This was a night the Celts could have folded their green tent and slinked home down, 3-1. They didn’t have Robert Williams due to knee irritation. They trailed by double digits in the second half. Everybody seemed to be in foul trouble when they started the fourth quarter, and superstar Jayson Tatum was still in a shooting slump.

Only six Boston players managed to score on this crucial night, and at times it seemed all of Boston was ready to give up and try to make ourselves feel better by complaining about officiating.

Then something happened. Everything happened. Al Horford happened — in Joe Hardy, Damn Yankees, Faustian bargain fashion.

Don’t-Call-Me-Average Al dunked on Giannis Antetokounmpo, flexed his veteran muscle, and erupted for 16 fourth-quarter points, making all six of his shots and virtually stomping on the Bucks logo. Horford finished with 30 points and eight rebounds.

“We love Al,” said Marcus Smart. “He’s the best veteran we’ve ever had. He understands what he brings to this game and what he brings to this team.”

Al Horford brings his arm down onto the face of Giannis Antetokounmpo after he dunked on him in the fourth quarter of Monday's game.Jim Davis/Globe Staff

Remember how some of us chuckled when Brad Stevens reacquired Horford in the offseason? It’s that same feeling we had when Chaim Bloom went out and got JBJ back last winter.

No one’s laughing now.

Horford was provoked when Giannis dunked and said something to him early in the third, the Bucks leading by 6. The Greek Freak picked up a technical for his trouble, and Horford looked especially annoyed.

“This is where the Bucks [expletive] up,” tweeted Anna Horford, Al’s sister. “I know that look. He was pissed.”

“I didn’t make out what he said,” humble Horford acknowledged after the game, “but the way he looked at me didn’t sit well with me. That got me going. At that point, something switched with me.”

In the already famous fourth, Horford postered Giannis with a dunk to tie the game, 80-80. Big Al drew a well-deserved technical for his antics, but nobody cared. Momentum had shifted and the Celtics were running downhill to victory.

“It was a big play, obviously,” Horford said. “Emotional for our group. A lot of emotions at that point.”

Horford’s fourth-quarter wingman was the aforementioned Tatum, who was 6 for 18 going into the quarter but finally got comfortable after three and three-quarter games. Tatum woke up and poured beer on the heads of the Deer that made Milwaukee famous.

Boston made an astounding 16 of 19 from the floor and 4 of 5 from international waters in the fourth quarter, outscoring the staggering Bucks, 43-28. The Champs were reduced to proverbial Deer in the headlights. The Bucks looked old. Giannis (34 points on 14-of-32 shooting) looked tired.

Not sure there has ever been anything like this. Probably not in the Russell-Cousy days. Maybe once or twice in the Larry-Kevin-Chief days. Certainly not in the KG-Pierce-Allen days. Red Auerbach must have been loving it from his perch highest above courtside.

Going into the night, the Celtics had to be thinking “must win.” Given that obvious fact, it’s hard to overstate how flat they were at the start. None of the energy from Saturday’s final quarter carried over and Boston fell behind, 21-11, in the first nine minutes.

Tatum, who shot only 35 percent (20 for 57) in Games 1-3, was 3-of-8 in the first half. He took only three shots in the first quarter — Daniel “Let it rain!” Theis took four! — and was tagged with two offensive fouls as Milwaukee led, 25-18, after one.

Al Horford scored 16 fourth-quarter points on Monday, at one point dunking on two-time MVP Giannis Antetokounmpo and leading the Celtics from 11 down with 15 minutes to play to a series tied at two games each.Jim Davis/Globe Staff

Had Dennis Eckersley been in Brewtown, he’d have said, “Yuk.” It was as if the Celts prepared by studying game film from their dark days of December.

It got better in the second as Horford and Jaylen Brown scored freely and the Celts dialed up their defense, but the Bucks still led, 48-47, at intermission. Milwaukee pushed the lead to 11 with 3:13 left in the third, and the Celtics were doing a lot of complaining about the zebras.

But the final quarter was all Boston. In somewhat historic fashion.

Playing for their playoff lives, the Celtics gutted the Bucks. It was a quarter of almost perfect basketball, something we haven’t seen maybe since Larry and Co. played a near perfect game against the Hawks in the playoff spring of 1986.

See you Wednesday.

The Celtic Revolution will be televised.