Many investors breathed a sigh of relief when the Federal Reserve announced it would leave rates unchanged on Wednesday. But Jim Cramer says the market isn't in the clear yet, and he's got a game plan prepared.

"We still need to face the U.K.'s big vote next week on whether to leave the European Union," the "Mad Money" host said.

Cramer was pleased that the Fed recognized that employment in the U.S. only grew by 38,000 jobs last month. It clearly knew that the economy is only slowly growing, and rate hikes shouldn't be on autopilot.

The uncertainty surrounding a Brexit was palpable in the market on Wednesday, and it prevented stocks from heading higher after the Fed announced its decision.

"I think more people are worried about this British vote than they would be otherwise, because they sure weren't worried a few weeks ago," Cramer said.

If a Brexit were to occur next week, Cramer shared his plan of action for investors. There could be a few days of opportunity for high-yield stocks like utilities and real estate investment trusts.

That could be a godsend.

This will be the chance to buy stocks like AT&T, General Mills, Bristol-Myers and Pfizer at very low prices. Cramer has his eye on Dominion Resources, American Electric Power, Consolidated Edison and Johnson & Johnson.

"When everyone is scared, you have to worry that you are going to miss the next buying opportunity," Cramer said.

Ultimately, Cramer is not calm about a Brexit. He simply wants investors to be ready for the next down day in the market. However, if the market is down 3 to 5 percent going into the vote, he recommended buying stocks that yield 3.5 percent ahead of the vote, and some after.

Cramer intentionally did not implement an aggressive approach due to oil inventory numbers dropping on Wednesday. He interpreted $45 as the support level to watch for crude, and knows it will be a huge variable to the stock market next week.

So, with the Fed out of the picture and oil and a Brexit in scope, Cramer will remain cautious until oil drops $3. That is when the market will fall 3 to 5 percent ahead of the vote in the U.K.

"That is when you put some of that mad money to work in higher yielding all domestic equities. That is the game plan, you just have to be ready to implement it," Cramer said.

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