说多少次会错一次呢？空口无凭，google it:

"Based on a true story"
http://www.google.com/search?q=%22Based+on+a+true+story%22
about 2,080,000

"Based on true story"
http://www.google.com/search?q=%22Based+on+true+story%22
about 248,000

"Based on true a story"http://www.google.com/search?q=%22Based+on+true+a+story%22
Results 1 - 10 of about 87 for "Based on true a story". (0.21 seconds)
Did you mean: "Based on true story"