The New England Patriots may not have played a particularly inspiring game against the Buffalo Bills, but they were still able to earn a 19-point road victory. As opposed to the last few games, the defense came up big in this one: not only did it keep Buffalo out of the end zone, it also produced two takeaways. The second of which — an 84-yard interception return touchdown by Devin McCourty — effectively iced the game in New England’s favor.

Let’s take a look at the film to find out how McCourty was able to score against the Bills and thus give the Patriots their first defensive touchdown since December 2015.

3-15-NE 30 (6:09) (Shotgun) D.Anderson pass short right intended for C.Clay INTERCEPTED by D.McCourty at NE 16. D.McCourty for 84 yards, TOUCHDOWN.

After the Patriots had just scored the game’s first touchdown to go up 18-6 in the fourth quarter, the Bills were able to drive all the way to New England’s 25-yard line. However, a series of negative plays — two incomplete passes and a false start penalty — forced the team into a 3rd and 15.

The home team approaches the clear passing situation with an 11-personnel group aligned in a 1x3 formation with quarterback Derek Anderson (#3) in the shotgun:

New England counters the Bills’ look with a rather standard two-deep safety package. The Patriots actually have a dime personnel group on the field, though, with three safeties and three cornerbacks playing man-to-man coverage across the board. At first glance, the alignment hints at either cover 2 or cover 4 — schemes in which safeties Devin McCourty (#32) and Duron Harmon (#21) would have responsibilities covering parts of the deep field.

However, McCourty does not sit back in his zone. Instead, he essentially plays a cover 3-like robber role in the middle of the field and patrols the underneath area:

Anderson originally looks left but the Bills’ quarterback quickly starts looking at tight end Charles Clay (#85), who originally aligned just on the outside of the right tackle. New England’s third safety — Patrick Chung (#23) — is in coverage of Clay and at the snap gives up inside leverage before playing a trail technique behind him. With the tight end a step ahead of the defensive back, Anderson takes his chance and fires a pass down the field.

The football, however, never reaches the intended target as McCourty comes up from the weak-side safety spot and undercuts the route to pick off the pass. With little else but 84 yards of green Buffalo turf in front of him, the speedster is able to take the interception to the house for six points:

McCourty plays the down perfectly. Not only does he give the initial impression of playing a deep-field role, he also quickly reacts to Anderson’s decision and uses his straight-line speed to make a play on the football. All in all, the 30-year old displays all of the traits that made him one of the league’s best safeties, even though he is having a comparatively rough year in 2018: instincts, intelligence, and athleticism.

While McCourty obviously deserves praise for the play, the rest of the defense cannot go unmentioned either. Anderson taking his chance with a pass to Clay would not have happened if the rest of the defense didn’t cover tightly: across the board, New England’s defenders were able to stick closely to their assignments. This would have made life hard for the quarterback even if he had not picked Clay as his target.

This level of execution in the secondary allowed the Patriots to not only get a stop on third down at a crucial moment in the game — even despite the pass rush not producing immediate pressure —, but also to turn it into points. The best case scenario.