In drama, we differentiate an act from a scene. A scene can be a part of an act and an act is named 幕 in Chinese whiel a scene is named 景.

However, people mixed up those two and sometimes a scene is mean with an act. So, in your context, you may call it 幕 --- 电影里的一幕.

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Note added at 6 hrs (2007-06-23 05:08:14 GMT)
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一“场”戏可分为好几“幕”，又在每一“幕”里可以分成好几个“景”，尤其拍电影时，分景是非常讲究的，否则根本无法分镜拍摄。

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Note added at 6 hrs (2007-06-23 05:14:01 GMT)
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"In a scene in the beginning of Back to the Future 2" could be translated into Chinese as follows:

在《回到未来 2》开头时的一幕里…

This translation misses nothing for it is commonly accepted when people mistake a scene for an act. But the setting of a scene is something quite different from a scene.