Cain waited as long as it takes to say "Uhh" before answering whether he could remember a worse inning.
"No," he said. "Nothing like this."
Cain yielded all nine runs, seven hits and two walks in that fourth inning. The last time a Giants starting pitcher gave up at least nine runs in an inning was Jon Cronin on Sept. 27, 1902.
The Cardinals won the last two games of the three-game, home-opening series in San Francisco, which was a rematch of the dramatic National League Championship Series that the Giants won last year.
"He just couldn't get out of that inning," Giants manager Bruce Bochy said. "He started to get the ball up a little bit. It was just one of those innings. We couldn't stop the bleeding."
It had been nine up, nine down through the first three frames with Giants ace Cain (0-1) cruising at his best. But, as the Giants have repeatedly said throughout their slow offensive start, baseball is sometimes about bounces. Once again, the Giants' opponent had them. In particular, it was one bounce, from the first batter in the fourth.
With San Francisco leading, 2-0, Jon Jay began the frame with a textbook single to center, and the one-hopper bounced off Angel Pagan's glove, allowing Jay to advance on the fielding error. Matt Carpenter singled to right, moving Jay to third. Allen Craig plated Jay with a sacrifice fly to make it 2-1. Carlos Beltran walked, then Yadier Molina singled before Matt Adams knocked a ground-rule double to deep right-center, scoring Carpenter and Beltran to give the Cardinals the lead, 3-2. Ty Wigginton followed with an RBI single. Pete Kozma then popped one just between backpeddling second baseman Marco Scutaro and right fielder Hunter Pence for another RBI single. Wainwright popped out on a bunt attempt, Jay walked, Carpenter singled in two more runs, and Bochy yanked Cain with the Cardinals ahead 7-2 after 3 2/3 innings.
Unfortunately for the Giants, reliever Jose Mijares kept the trend going. He hit his first batter, Craig, then Beltran hit a two-run single to make it 9-2, with both runs attributed to Cain. Mijares struck out Molina to finally end the inning, eliciting some of the loudest cheers since Pagan's RBI double and Pablo Sandoval's RBI single, both in the third inning.
Cain "was missing the plate more than he usually does," catcher Buster Posey said. "They put some good swings on him, too."
In all, 14 St. Louis batters appeared in the fourth inning and collected eight hits. Cain matched his career high for earned runs allowed, also doing so against the Cardinals on April 18, 2008. The nine runs allowed in the fourth were the most the Giants have yielded since Aug. 5, 2004, against the Cincinnati Reds.
To the Giants fans' credit, few left the ballpark immediately after the fourth inning, which they could have, as 40,000 of them already had replica World Series rings and the memories of a special pregame ring ceremony.
"The game does put a little bit of a damper" on the festivities, Pence said. "But in the long run, we'll remember the day we got the rings."
Meanwhile, Cardinals right-handed ace Adam Wainwright (1-1) only struggled in the third and in his seven innings allowed two runs, seven hits and no walks while striking out six.
The Cardinals added two unearned runs in the eighth inning and three more tallies in the ninth. Six different Cardinals had at least two RBIs, while Pagan and Sandoval were the only Giants to have at least two hits. San Francisco did get a reason to cheer in the ninth when Brandon Belt recorded his first hit of the season, Nick Noonan got his first Major League hit and Guillermo Quiroz drove in his first run as a Giant with a single.
Cain, who in 2012 reached career bests in wins (16), ERA (2.79) and strikeouts (193), recognized that these days -- or even innings -- happen to the best of them.
"In general, baseball has a way of humbling guys," he said. "The baseball gods find a way to even it out."
Whether it is when they next see San Francisco on May 31, or maybe even yet again in October, the Cardinals believe they will see Cain shine again.
Said St. Louis' Matt Carpenter, who is now 6-for-7 lifetime against Cain:
"I don't really know what to say about it, other than I know he's an outstanding pitcher, one of the best in the game. It's crazy the way it's worked out. It's not like I have a secret or anything. Every at-bat I've had against him has been real tough; I just try to battle."